Silver-containing photothermographic imaging materials (that is, thermally developable photosensitive imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced. In most instances, the source of reducible silver ions is an organic silver salt in which silver ions are complexed with organic silver coordinating ligands.
Thermographic materials are similar in nature except that the photocatalyst is omitted and imaging and development are carried out simultaneously using a thermal imaging means. Such materials also include an organic silver salt that provides reducible silver ions required for imaging.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called “instant photography,” the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74–75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94–103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
As noted above, non-photosensitive sources of reducible silver ions are critical to the imaging mechanism of both photothermographic and thermographic materials. Various organic silver salts are useful for this purpose including silver carboxylates (both aliphatic and aromatic), silver salts of nitrogen-containing heterocyclic compounds, silver sulfonates, and many others known in the art as described for example in U.S. Pat. No. 6,576,410 (Zou et al.).
Aqueous-based photothermographic materials have been known for many years in which the imaging components and binders are formulated in and coated from solvents comprising primarily water. It has been necessary in designing such materials that the various imaging components be compatible with water and other water-soluble or -dispersible components. Silver benzotriazole has been found particularly useful in aqueous-based materials because of the hydrophilic nature of silver benzotriazole crystal surfaces and its compatability with most water-soluble binders.
One challenge in photothermographic materials is the need to prevent image artifacts known as “black spots” after thermal development. Black spots are believed to be caused by crystallization of active toners or agglomeration of toner particles during dispersion, melt preparation, coating, and drying of thermographic and photothermographic materials. Upon thermal development, the local concentration of active toner where these toner particles reside is so high as to cause spontaneous development in the non-imaged areas, resulting in high-density black spots.
Another challenge in photothermographic materials is the need to improve their stability after use. This is referred to as “Archival Stability” or “Dark Stability.” It is desirable that the Dmin not increase, and that the Dmax, tint, and tone of the image not change.
A further challenge in photothermographic materials is the need to improve their stability at ambient temperature and relative humidity during storage prior to use. This stability is referred to as “Natural Age Keeping” (NAK) or as “Raw Stock Keeping” (RSK). It is desirable that photothermographic materials be capable of maintaining imaging properties, including photospeed and Dmax, while minimizing any increase in Dmin during storage periods. Natural Age Keeping is a problem especially for photothermographic films compared to conventional silver halide photographic films because, as noted above, all the components needed for development and image formation in photothermographic systems are incorporated into the imaging element, in intimate proximity, prior to development. Thus, there are a greater number of potentially reactive components that can prematurely react during storage.
Mercaptotriazoles have been described for use as toners in photothermographic materials in U.S. Pat. No. 3,832,186 (Masuda et al.), U.S. Pat. No. 4,201,582 (White), U.S. Pat. No. 4,105,451 (Smith et al.), and U.S. Pat. No. 6,713,240 (Lynch et al.). The mercaptotriazoles described in U.S. Pat. No. 6,713,240 are especially useful toners (or toning agents) and development accelerators for photothermographic materials.
Mercaptotriazoles suitable for thermally developable imaging materials often have poor water solubility and cause undesirable precipitation when added to aqueous-based imaging formulations, thereby adversely affecting coating quality and density uniformity.
Moreover, the presence of such toners in photothermographic materials during storage before use also may accelerate the increase in Dmin. Thus, photothermographic materials that include large quantities of mercaptotriazole toners to accelerate the development reaction may be susceptible to keeping problems, leading to reduced “NAK.”
U.S. Pat. Nos. 6,576,414 (Irving et al.) and 6,548,236 (Irving et al.) describe both color and black-and-white photothermographic materials containing core/shell particles having two or more different organic silver salts. The particles function as silver sources.
There remains a need to effectively incorporate specific organic silver salts and mercaptotriazole toners into aqueous-based photothermographic imaging formulations and materials so that formation of black spots is reduced and sensitometric properties are not changed during Natural Age Keeping, and so that Archival Stability is improved, all without sacrifice of desired photospeed and other sensitometric properties.